1. Irina Bavykina, MPI f. Physik
Investigation of ZnWO4 crystals as an absorbers in the CRESST Dark Matter search
detector concept
active background discrimination
motivation for ZnWO4 absorber
results
21/07/06
Irina Bavykina, MPI f. Physik

2. Motivation for Dark Matter searches on different astrophysical scales
21/07/06
Irina Bavykina, MPI f. Physik

3. WIMP direct detection requirements
elastic scattering off nucleus: c
low energy transfer
to nucleus (< 40 keV)
scattering events very rare:
< 0.1 / (kg day)
recoil
best choice:
low
temperature
calorimeters
low energy threshold
high target mass
radiopure materials
background discrimination
21/07/06
Irina Bavykina, MPI f. Physik

4. Irina Bavykina, MPI f. Physik
Detector concept
calorimeter model
simple power-flux model
heat bath
absorber
temperature sensor
pulse height
time ΔT
τdec
ΔT ~ ΔE/C
τdec ~ C/G
reality is more complicated:
no thermal equilibrium
decoupling of subsystems
21/07/06
Irina Bavykina, MPI f. Physik

5. Irina Bavykina, MPI f. Physik
Readout of SPT
SQUID output [V]
Temperature [mK]
21/07/06
Irina Bavykina, MPI f. Physik

6. Main limitation: radioactive background
active discrimination between electron recoils and nuclear recoils
reflecting cavity
silicon absorber
scintillating dielectric absorber χ χ
21/07/06
Irina Bavykina, MPI f. Physik

7. Irina Bavykina, MPI f. Physik
Quenching factor
Quenching Factor =
light output from electron recoil
light output from nuclear recoil
Irradiation with e- and 
Irradiation with e-,  and n
Energy in light channel [keVee]
Energy in phonon channel [keV]
Energy in phonon channel [keV]
21/07/06
Irina Bavykina, MPI f. Physik

8. Irina Bavykina, MPI f. Physik
Open detector module
21/07/06
Irina Bavykina, MPI f. Physik

9. Motivation for different target materials
discrimination technique can work with different dielectric scintillators
BGO, BaF2, PbWO4, Al2O3, CaWO4, CdWO4, ZnWO4, etc.
comparision of rates and spectra in different target materials
to varify positive signal
Important characteristics:
light yield at low temperature (defines discrimination threshold)
quenching factor (determination of nuclear recoil acceptance region )
high atomic mass (to take an advantage of large scalar cross section)
surface properties, radio purity,
phonon propogation properties
ZnWO4
21/07/06
Irina Bavykina, MPI f. Physik

10. Irina Bavykina, MPI f. Physik
Properties of ZnWO4
Properties
CaWO4
ZnWO4
Density (g/cm3)
6.06
7.87
Melting point (°C)
1570
1200
Structure type
Sheelite
Wolframite
Emission peak at 300K (nm)
420
480
Emission peak at 8K (nm)
510
(ACa=40, AZn=65)
19.3% more W nuclei !
21/07/06
Irina Bavykina, MPI f. Physik

11. Light yield (H. Kraus et al, 2005)
temperature dependence of the integrated luminescence yield
Temperature [K]
Intensity, a. u.
Integrated light yield of ZnWO4 at 8K is ~ 10% higher
21/07/06
Irina Bavykina, MPI f. Physik

12. He3 / He4 dilution refrigerator
21/07/06
Irina Bavykina, MPI f. Physik

13. He3 / He4 dilution refrigerator
Constant concentration of 6.5% of He3 in He4 at saturated vapor pressure even at 0K! This is finite solubility phenomenon.
21/07/06
Irina Bavykina, MPI f. Physik

14. Irina Bavykina, MPI f. Physik
ZnWO4 phonon detector
Heat sink temperature [mK]
SQUID output [V]
ZnWO4
W-SPT
SiO2
ZnWO4 crystal with
W - SPT
110
115
120
130
140
125
145
150
135 70 30 60 50 40 20 10 80
Energy [keV]
Counts
SiO2 diffusion barrier:
Tc≈ 7 to 25 mK (adjustable)
TD (°C)
510
680
740
TC (mK)
200 55 22
Energy resolution:
ΔEFWHM= (2.26 ± 0.14) E=122 keV
21/07/06
Irina Bavykina, MPI f. Physik

15. Measurement of the quenching factor of ZnWO4
Irradiation with e- and 
Irradiation with e-,  and n
Electron recoils
Electron recoils
Pulse height in light detector [keVee]
Pulse height in light detector [keVee]
Nuclear recoils
Pulse height in phonon detector [keV]
Pulse height in phonon detector [keV]
21/07/06
Irina Bavykina, MPI f. Physik

16. Irina Bavykina, MPI f. Physik
Results
Light to phonon ratio
Energy in phonon detector [keV]
21/07/06
Irina Bavykina, MPI f. Physik

17. Irina Bavykina, MPI f. Physik
ZnWO4 quenching factor
Energy range [keV]
Quenching factor
of neutrons
10 to 20
38.5 ± 1.6
20 to 40
12.5 ± 0.4
40 to 60
10.5 ± 0.1
60 to 80
10.7 ± 0.1
80 to 100
10.0 ± 0.1
100 to 120
11.1 ± 0.1
21/07/06
Irina Bavykina, MPI f. Physik

18. Irina Bavykina, MPI f. Physik
Conclusions
quenching factor of neutrons with ZnWO4 was measured for the first time
measured light output below 150 keV is highly non-linear (could limit active background suppresion potential)
ZnWO4 appeared to be suitable target material for CRESST
ZnWO4 can be a complement to CaWO4 for the first multi-target detector capable of WIMP identification through material signature
21/07/06
Irina Bavykina, MPI f. Physik